1. Technical Field
The invention relates to an optical scanning apparatus which makes a light beam scan on a surface-to-be-scanned in a main scanning direction, a control method of such an optical scanning apparatus, and an image forming apparatus which executes image formation using such an optical scanning apparatus.
2. Related Art
In an optical scanning apparatus of this type, a light source and a deflector are provided, and the light beam emitted from the light source is deflected by the deflector, thereby scanning the surface-to-be-scanned with the deflected light beam in a main scanning direction. Further, like an optical scanning apparatus described in JP-A-2002-182147 for example, in order to achieve the size reduction and speed-up of the deflector, an apparatus has heretofore been proposed which employs an oscillating deflection mirror as the deflector. That is, the apparatus is structured that a deflection mirror supported by a torsion bar is sinusoidally oscillated and a light beam emitted from a light source is reflected by a surface of the deflection mirror, whereby the light beam scans a surface-to-be-scanned such as a surface of a latent image carrier in a main scanning direction.
Further, in the optical apparatus described in JP-A-2002-182147, in order to scan the surface-to-be-scanned at a constant speed with the light beam deflected by the deflection mirror which oscillates sinusoidally as described above, a scanning optical system which has an arcsine characteristics is used. That is, the larger the incident angle to the scanning optical system of the light beam deflected by the deflection mirror which oscillates sinusoidally becomes, the slower the angular velocity of the incident angle becomes. Therefore, in the case where an orthoscopic scanning optical system is used for instance, the longer the distance (image height) from the optical axis in the main scanning direction becomes, the slower the scanning speed of the light beam on the surface-to-be-scanned becomes. Consequently, in order to compensate the decrease of the scanning speed at a position the image height being large, a scanning optical system which has an arcsine characteristics is used in which the output angle of the light beam increases more rapidly compared to the increase of the incident angle.